Move LogicalDisplay.isEnabled calculation to DPC.

Move the logicalDisplay.isEnabled() check from being a one-off
check in DisplayManagerService to being part of the display-power
calculation in DisplayPowerController.

Also:
- Change initial value of mDisplayState to UNKNOWN so that we dont
send displayState requests until the display's power state has been
initialized by DisplayPowerController.
- Sent the displayState to SurfaceControl when the
previous state was UNKNOWN. This is needed to ensure the second
display on multi-display devices can be set to OFF by default.

Test: Manually verify no OFF state is sent during boot-up on
single-display devices
Test: atest com.android.server.display
Bug: 181314849
Bug: 178713651

Change-Id: If3b0880b173bc11974f1a61d159d7a4ca9a0c159